This invention relates to a combustor for a gas turbine and more particularly, to a combustor of the premix combustion-type and a method of operating such a combustor.
Most of conventional combustors of the general type employ a two-stage combustion system to reduce production of NOx. More specifically, in such a combustor, a diffusion combustion is effected at one end of a combustion cylinder, at the head of the combustor, for the purpose of stabilizing a flame whereas, a premix combustion, highly effective in reducing NOx, is effected downstream of the one end of the combustion cylinder.
Such a combustor is disclosed, for example, in U.S. Pat. No. 4,292,801. More specifically, this conventional combustor comprises a first-stage diffusion combustion burner mounted on a head of the combustor, and a second-stage diffusion combustion burner extending from the combustor head toward a central portion of a combustion chamber. The diameter of the combustion chamber is reduced or constricted in the vicinity of an outlet of the second-stage burner. At the start of combustion, fuel is supplied to the first-stage burner and is ignited so as to effect a diffusion combustion in a first-stage combustion chamber. Then, as the combustion load increases, fuel is introduced into the second-stage burner so as to effect a diffusion combustion in a second-stage combustion chamber. Then, simultaneously when the first-stage fuel is transferred to the second-stage burner, the first-stage burner is extinguished (turned off), and then fuel is again charged into the first-stage burner. At this time, the first-stage combustion chamber serves as a premix chamber for mixing the fuel and the air together. Therefore, at the time of a high-load combustion, a fuel-air premixture from the first-stage combustion chamber is burned by the heat source of the second-stage burner so as to continue the combustion.
In the combustor of such a construction, premix combustion is carried out mainly when the combustor is operated under a load higher than a predetermined level, and therefore this combustor is very effective in reducing NOx. This type of combustor is satisfactory in that during a high-load operation, it discharges small amounts of NOx and uncombusted substances such as CO, which means that the combustion is sufficiently effected in the combustion chamber, and therefore very little of the combustible fuel-air mixture is discharged in an uncombusted condition from the combustor. However, during a low-load operation of the combustor, that is, under a condition in which the fuel-air mixture is lean, the cooling air enters about the wall surface of the combustion chamber, so that the amount of discharge of unburned substances such as CO tends to increase.